Cell walls are found in both prokaryotes and eukaryotes. Bacteria, archaea, plants, fungi, and many algae all have cell walls, though the walls in each group are made of very different materials. The major group of life that consistently lacks a cell wall is animals. That shared trait across such different organisms, built from such different chemistry, is one of the more interesting stories in biology.
Bacterial Cell Walls and Peptidoglycan
The cell wall most people encounter in a biology class is the bacterial version, built from a material called peptidoglycan. This is a mesh-like scaffold that wraps around the bacterial cell membrane, giving the cell its shape and protecting it from bursting under internal pressure.1PubMed Central. Peptidoglycan: Structure, Synthesis, and Regulation Peptidoglycan consists of sugar chains linked together by short chains of amino acids, forming something like a woven net that surrounds the entire cell.2FEMS Microbiology Reviews. Peptidoglycan structure and architecture
How thick that net is depends on the type of bacterium. Gram-positive bacteria have a thick peptidoglycan layer, many times thicker than what you find in Gram-negative species. Gram-negative bacteria compensate with an additional outer membrane that contains lipopolysaccharide, a molecule that plays its own protective role. Threaded through the thick walls of Gram-positive species are long charged polymers called teichoic acids, which contribute to the wall’s overall function.3PubMed Central. The bacterial cell envelope This Gram-positive versus Gram-negative distinction has enormous practical consequences, particularly for how antibiotics work, a point worth returning to later.
Archaea Build Their Walls Differently
Archaea are prokaryotes, but their cell walls look nothing like those of bacteria. The defining difference is that archaea lack peptidoglycan entirely.4PubMed Central. Archaeal S-Layers: Overview and Current State of the Art Instead, nearly all known archaea are coated in a surface layer, or S-layer, made of protein that self-assembles into a crystalline sheet around the cell.5PubMed Central. The archaeal cell envelope
Some archaea do produce sugar-based polymers for their walls, including one called pseudomurein, which resembles peptidoglycan in broad outline but differs enough chemically that antibiotics designed to target bacterial peptidoglycan have no effect on it.5PubMed Central. The archaeal cell envelope This is a useful fact to keep in mind: “prokaryote” does not mean “peptidoglycan cell wall.” The two major branches of prokaryotic life solved the problem of building a protective envelope in fundamentally different ways.
Plant Cell Walls Are Cellulose-Based
When you shift to eukaryotes, the chemistry changes again. Plant cell walls are built primarily from cellulose, hemicellulose, pectin, lignin, and some proteins.6PubMed Central. Structure, Modification Pattern, and Conformation of Hemicellulose in Plant Biomass Cellulose fibers provide tensile strength, hemicelluloses cross-link those fibers into a tighter network, pectins form a gel-like matrix that fills in the gaps, and lignin adds rigidity, particularly in wood and other tissues that need to be stiff.
Plant cells actually build their walls in stages. A thin, flexible primary wall goes up first, allowing the cell to grow and expand. Once the cell reaches its final size, many plant cells deposit a secondary wall inside the primary one. This secondary wall is denser and more rigid, with cellulose fibers packed tightly alongside hemicellulose and lignin.7PubMed Central. Cellulose-Hemicellulose-Lignin Interaction in the Secondary Cell Wall of Coconut Endocarp Wood is essentially secondary cell wall material, which gives you a sense of how strong this structure can become.
One thing that surprises people about plant cell walls is that they are not sealed barriers. Plants maintain channels called plasmodesmata that punch through the walls between neighboring cells, creating a continuous cytoplasmic network.8PubMed Central. Communicating Across Cell Walls: Structure, Evolution, and Regulation of Plasmodesmatal Transport in Plants These channels are not passive holes. They are lined with membrane, their openings are actively regulated by signaling cascades, and they allow cells to exchange small molecules, proteins, and possibly even large RNA molecules.9PubMed. Plasmodesmata and intercellular molecular traffic control Different cell types host different numbers and forms of plasmodesmata, effectively controlling which molecules flow where and how fast.10PubMed Central. Cell-to-cell transport of proteins and fluorescent tracers via plasmodesmata during plant development So a plant cell wall is both a structural shield and a communication platform.
Fungal Cell Walls Run on Chitin
Fungi are eukaryotes, but their walls are built from yet another set of materials. The backbone is chitin, the same polymer that makes up the exoskeletons of insects and crustaceans, along with various forms of glucan. Cross-links between chitin and glucan, mediated by specific enzymes, give fungal walls their strength and flexibility, and these linkages are actively remodeled as the fungus grows or responds to stress.11PubMed. Strengthening the fungal cell wall through chitin-glucan cross-links: effects on morphogenesis and cell integrity
The fact that fungal cell walls are chemically distinct from both bacterial and plant walls matters a great deal for medicine. Fungal infections in humans are notoriously hard to treat because fungi, like us, are eukaryotes. Many drugs that work against bacteria are useless against fungi, and many drugs that hurt fungi also risk hurting human cells. The fungal cell wall is one of the few targets that is present in fungi and completely absent in animal cells, making it a prime focus for antifungal drug development.12PubMed Central. Targeting the fungal cell wall: current therapies and implications for development of alternative antifungal agents
Algae and Other Microbial Eukaryotes
Algae are a diverse group, and their cell walls reflect that diversity. Many green algae have cellulose-based walls, broadly similar to plants. Brown and red algae incorporate other polysaccharides. But the most striking variation comes from diatoms, single-celled photosynthetic organisms that build their walls out of silica, essentially glass. These silica walls, called frustules, come in intricate geometric patterns and are one of the more visually remarkable structures in biology.13Algae. Differential expression of silicon transporter genes in frustule formation of the marine diatom Thalassiosira eccentrica LIMS-PS-3165
The sheer range of cell wall materials across eukaryotic microorganisms, from cellulose to silica to chitin to unusual polysaccharides, underscores a broader point. Cell walls are not defined by a single chemical recipe. They are defined by a function: providing structural support and protection outside the cell membrane. Different lineages have arrived at that function independently, using whatever building blocks their biochemistry had available.
Who Does Not Have a Cell Wall
The most prominent group without cell walls is animals. No animal cell at any stage of development produces a cell wall. Animal cells rely instead on internal scaffolding (the cytoskeleton) and an extracellular matrix of proteins and sugars for structural support and tissue organization. This absence is part of what allows animal cells to be flexible, mobile, and capable of rapidly changing shape, traits that matter for muscle contraction, immune cell migration, and embryonic development.
Many single-celled eukaryotes (protists) also lack true cell walls, though some have other rigid or semi-rigid coverings. Amoebas, for instance, have no wall at all. The picture across eukaryotes is a patchwork: plants, fungi, and many algae have walls; animals and many protists do not.
Prokaryotes That Break the Rule
If you assume all prokaryotes have cell walls, mycoplasmas are the exception that disproves it. Mycoplasmas are bacteria, firmly prokaryotic, yet they naturally and permanently lack a cell wall.14PubMed Central. Survival strategies of mycoplasmas: the critical role of post-translational modifications They get by with just a cell membrane, which makes them among the smallest self-replicating organisms known. Their cell division machinery has adapted to work without any wall synthesis at all, and researchers have studied them as model systems for understanding how cells can divide in the absence of a rigid outer structure.15PubMed. Membrane Binding and Cholesterol Sensing Motif in Mycoplasma genitalium FtsZ: A Novel Mode of Membrane Recruitment for Bacterial FtsZ
There is also a temporary version of wall-less bacteria. Under certain stressful conditions, including exposure to antibiotics that attack peptidoglycan, ordinary walled bacteria can shed their cell walls and enter what is called an L-form state. These L-forms can grow and divide as long as their environment protects them from the osmotic pressure that would otherwise destroy a wall-less cell.16PubMed Central. Cell wall-deficient, L-form bacteria in the 21st century: a personal perspective Once the stressful conditions pass, many L-form bacteria revert to their normal walled state.17PubMed Central. Bacterial L-forms: Key Mechanisms of Drug Resistance, Disease Recurrence, and Immune Evasion
L-forms matter for medicine because they represent a potential escape route from antibiotics. A bacterium that sheds its wall becomes invisible to drugs designed to target wall synthesis. If it can survive long enough in a protected environment inside the body and then rebuild its wall later, it could cause a relapse of infection after treatment ends.17PubMed Central. Bacterial L-forms: Key Mechanisms of Drug Resistance, Disease Recurrence, and Immune Evasion
Why Cell Walls Are a Prime Antibiotic Target
The reason so many antibiotics go after the bacterial cell wall is straightforward: animal cells do not have one. Any drug that disrupts peptidoglycan synthesis hits bacteria hard while leaving human cells alone, at least in principle. Penicillin, the first widely used antibiotic, works exactly this way, blocking enzymes that bacteria need to build and maintain their peptidoglycan scaffold.18PubMed Central. Agents Targeting the Bacterial Cell Wall as Tools to Combat Gram-Positive Pathogens
The antibiotics in this category are not all doing the same thing at a molecular level. Some block the enzymes directly. Others, interestingly, work by binding tightly to the building-block molecules that bacteria need for wall assembly, effectively sequestering the raw materials before the enzymes can use them.19PubMed. Targeting membrane-bound bacterial cell wall precursors: a tried and true antibiotic strategy in nature and the clinic Vancomycin, one of the last-resort antibiotics used against drug-resistant infections, is a classic example of this substrate-binding strategy.
A similar logic applies to antifungal drugs. One important class, the echinocandins, targets the synthesis of a glucan component found in fungal cell walls but absent from human cells.20PubMed Central. Echinocandins – structure, mechanism of action and use in antifungal therapy Because the target exists only in fungi, these drugs tend to have fewer side effects than older antifungals that attack shared eukaryotic pathways. The cell wall’s absence in animals is, in a real sense, what makes many of our best antimicrobial drugs possible.
Cell Walls and Turgor Pressure
One of the main jobs of a cell wall, in any organism that has one, is resisting turgor pressure. Turgor is the outward force exerted on the wall by water flowing into the cell through osmosis. Without a wall, a cell in a low-solute environment would swell and burst. With a wall, the cell pushes back, reaching an equilibrium that keeps the cell turgid and structurally sound.21PubMed Central. On the osmotic pressure of cells
In bacteria, this relationship between turgor and the wall turns out to be a two-way street. Research on E. coli has shown that turgor pressure increases with growth rate, and that turgor directly controls how fast the cell builds new wall material.22PubMed Central. Bacterial cell wall biosynthesis is controlled by growth rate dependent modulation of turgor pressure in E. coli A faster-growing cell has higher internal pressure, and that pressure itself signals the wall-building machinery to ramp up production. The wall is not just a passive shell; it is part of a feedback loop that responds to how the cell is growing.
Cell Walls in Symbiosis
Cell walls also serve as the interface where organisms negotiate their relationships with each other. In mycorrhizal symbiosis, the widespread partnership between plant roots and soil fungi, both partners remodel their cell walls at the point of contact. The plant modifies its wall to allow the fungus entry, and the fungus adjusts its own wall as it penetrates root tissue, creating a specialized compartment where nutrients are exchanged.23PubMed Central. Cell wall remodeling in mycorrhizal symbiosis: a way towards biotrophism The composition and organization of cell walls at this interface has been studied extensively because it reveals how two very different organisms, a plant with cellulose-based walls and a fungus with chitin-based walls, manage to coexist and cooperate at the cellular level.
Pathogens use a similar logic in reverse. Many plant diseases involve fungi or bacteria that produce enzymes to break down the host’s cell wall, opening a path for invasion. The wall is a battlefield as much as it is a shelter.
Cell Wall Biomass as a Resource
Plant cell wall material is the most abundant biological resource on the planet, and that abundance has made it a major target for sustainable energy and industry.24The Cell Surface. Powerful cell wall biomass degradation enzymatic system from saprotrophic Aspergillus fumigatus Converting cellulose and hemicellulose into biofuels requires breaking down the wall’s tightly woven structure, which is chemically difficult precisely because the wall evolved to resist degradation. In nature, fungi like Aspergillus fumigatus have evolved powerful suites of enzymes that can digest cellulose, hemicellulose, chitin, and other wall polymers, and researchers have been studying these enzyme systems for their potential industrial applications, including their tolerance for high temperatures and acidic conditions.24The Cell Surface. Powerful cell wall biomass degradation enzymatic system from saprotrophic Aspergillus fumigatus
How Researchers Study Cell Wall Mechanics
Measuring the physical properties of something as small as a cell wall requires specialized tools. One of the most revealing is atomic force microscopy, which uses a tiny probe to press into the cell surface and measure how stiff or flexible the material is at each point. This technique has been used on living plant cells to map stiffness at different depths within the wall, showing how the mechanical properties change from the outer surface inward and shift as the cell grows.25PubMed Central. Atomic force microscopy stiffness tomography on living Arabidopsis thaliana cells reveals the mechanical properties of surface and deep cell-wall layers during growth When combined with spectroscopy that reveals the wall’s chemical composition, these measurements allow researchers to connect specific building materials to specific mechanical behaviors.26PubMed. Atomic Force Microscopy to Study Cell Wall Mechanics in Plants One challenge is that turgor pressure from inside the cell affects the readings, so some protocols work with thin sections of embedded tissue rather than live cells, eliminating turgor’s influence and revealing the wall material’s intrinsic stiffness.
How Hooke’s Cells Got Their Name
The word “cell” in biology traces back to Robert Hooke, the seventeenth-century microscopist who examined thin slices of cork under a microscope and saw a honeycomb of tiny compartments. What Hooke was actually looking at were the remnants of dead plant cell walls, the rigid cellulose boxes left behind after the living contents had dried away. Modern accounts sometimes oversimplify Hooke’s contribution, claiming he used “pore” and “cell” interchangeably, when in fact his terminology was more precise than is commonly recognized.27Notes and Records: the Royal Society Journal of the History of Science. The cells of Robert Hooke: pores, fibres, diaphragms and the cell theory that wasn’t The irony is that the very feature that made cells visible to the first person who ever saw them, the rigid wall that held its shape even in death, is a feature that many types of cells, including all of our own, do not possess.